Alpha release 3.0

What's Changed

• Fix isel on Dfsu 3d single timestep. by @ecomodeller in #828
• Simplify Dataset by @ecomodeller in #829
• Simpler dfs0 by @ecomodeller in #831
• Upgraded syntax by @ecomodeller in #832
• Change dfs datatype by @otzi5300 in #830
• Linespectra inheritance by @ecomodeller in #836
• Interpolation ids and weights always go together by @ecomodeller in #835
• Mypy upgrade by @ecomodeller in #838
• Generic derived variables by @ecomodeller in #839
• Feature/write_spectral_dfsu by @ecomodeller in #837

Full Changelog: v2.6.0...v3.0.0a0